That would mark the fastest quarterly growth since a 4.1% spurt in the last three months of 2011 and the best third-quarter performance in five years.

Why the increase? Fresh data show that companies spent more to build up inventories in the third quarter than originally estimated and the U.S. trade deficit narrowed. Lower net imports increase GDP.

“We expect both to provide a small boost to overall growth, having been modest drags in the first estimate,” economists at Barclays Capital said in a research note.

The U.S. releases two updates of each quarterly GDP report after first version comes out. The second version incorporates new data not available on the first go-around.

Also on Thursday, the Labor Department will release weekly jobless claims and the National Association of Realtors will issue pending home sales for October. Claims are predicted to decrease by 20,000 to 390,000 in the week ended Nov. 24, the MarketWatch survey shows. Claims shot up several weeks ago after the devastation caused by Hurricane Sandy and economists have been discounting the data until the effects of the storm fade. It’s unclear how long that will take.

Cliff to trump better growth

Yet even if the faster pace of third-quarter growth materializes, it probably won’t have much impact on investors, businesses and consumers. Markets have already built in expectations of a better GDP number.

What’s more, the revised third-quarter numbers won’t offer much clue as to where the economy is headed. In the final three months of the year, for example, growth is projected to slow to a 1.6% rate, the MarketWatch forecast shows.

That’s the scary-sounding term used to describe the combination of $500 billion-plus in tax increases and deep government spending cuts set to occur after Jan. 1. Democrats and Republicans have to strike a compromise politically painful to one or both sides to avert a fall off the cliff.

Growth in the fourth quarter could downshift more sharply if it looks like Washington won’t reach a deal. Companies are more likely to put off some major investments if the uncertainty gets worse. So far, they have continued to invest at a modest pace despite the cliff-hanger taking place in the nation’s capitol.

What’s more, the composition of third-quarter GDP raises questions about whether such a pace of growth is sustainable.

Lance Roberts, host of the financial website StreetTalkLive, points out that a huge bump in defense spending gave third-quarter growth a big pop.

Another questions surrounds the role of inventories. Companies typically boost their stock of goods on hand in anticipation of rising demand for their products — a bullish signal about the future.

Yet inventories can also rise unexpectedly because sales don’t materialize as expected. When that happens, companies cut back on production and that contributes to a smaller growth in the next quarter or two.

It may take several months before economists know whether the increase in inventories, mainly at U.S. factories, was intended or accidental. Consumer confidence has risen steadily and retail spending is solid, but there’ no indication that sales are about to sizzle.

The decline in the U.S. trade deficit, meanwhile, was not the result of U.S. companies sharply boosting sales in overseas markets where growth has slowed. Instead Americans purchased fewer imported goods —mainly gasoline — in the third quarter compared to the prior period.

Economists will pay to close another number in the GDP report: final sales of domestic product, a category that omits inventories and gives a better idea of underlying domestic demand for goods and services.

Final sales initially were reported to have risen 2.1% in the third quarter, but economists say it could be revised to 2.5% or somewhat higher.

The major obstacle to the forecast for higher third-quarter growth is the shipment of capital goods — things like factory equipment that are used to make other products for sale to consumers and businesses.

Shipments were revised lower in September in a sign that business investment was weaker in the third quarter than initially forecast.

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